In a case where an external gear having a number of teeth differing from the number of teeth of an internal gear is disposed inside the internal gear while the external gear is in a meshed state with the internal gear and the external gear is made to revolve orbitally around an axis line of the internal gear while maintaining the meshed state, the external gear rotates with respect to the internal gear. Inner meshing planetary gear type reduction gear transmissions utilizing this mechanism have been developed, and an example thereof is disclosed in Japanese Patent Application Publication No. 2000-154849. This reduction gear transmission comprises an internal gear, an external gear, a carrier, a crankshaft, and a bearing.
The external gear has a number of teeth differing from a number of teeth of the internal gear, and is surrounded by the internal gear. The external gear rotates while revolving orbitally with respect to the axis line of the internal gear while maintaining a meshed state with the internal gear. A plurality of through holes are formed in the external gear at positions offset from a center of the external gear.
A carrier is supported such that the carrier is capable of rotating with respect to the axis line of the internal gear. Further, a part of the carrier passes through some of the through holes formed in the external gear. A space is maintained between the through hole and the carrier. As a result, the carrier allows the external gear to revolve orbitally with respect to the axis line of the carrier, and the carrier rotates following the rotation of the external gear.
The crankshaft comprises a shaft part capable of rotating with respect to the carrier, an input gear fixed to the shaft part and capable of causing the shaft part to rotate, and an eccentric disk fixed to the shaft part and having its center at a position offset from an axis line of the shaft part. The eccentric disk of the crankshaft is fitted into the other through holes (the through holes into which the carrier is not inserted) of the external gear.
The bearing is arranged between the carrier and the shaft part of the crankshaft. Further, the bearing supports the shaft part of the crankshaft such that the shaft part is capable of rotating with respect to the carrier.
The eccentric disk of the crankshaft rotates eccentrically following the rotation of the input gear of the crankshaft. When the eccentric disk of the crankshaft rotates eccentrically, the external gear rotates while revolving orbitally with respect to the internal gear. When the external gear rotates, the carrier rotates with respect to the internal gear.
In the inner meshing planetary gear type reduction gear transmission, the crankshaft must be capable of rotating with respect to the carrier. In the conventional reduction gear transmission described above, the shaft part is formed so as to extend from the eccentric disk toward both sides in an axial direction of the crankshaft. This shaft part is supported with respect to the carrier by cylindrical roller bearings. That is, in the conventional reduction gear transmission, the shaft part is provided that extends from the eccentric disk toward both sides in the axial direction, and this shaft part must be supported by the bearings. As a result, the crankshaft becomes longer in its axial direction. The reduction gear transmission consequently becomes longer in its rotational axis line direction.